1. Field of the Invention
This invention relates to a superlattice structure with an artificially controllable potential energy barrier.
2. Description of the Prior Art
For light emitting semiconductor devices, such as semiconductor lasers, light emitting diodes, etc., it is required to effectively confine injected carriers, such as electrons or holes, contributing to the light emission within an active region. In conventional light emitting semiconductor devices, therefore, a double heterostructure as shown in FIG. 1 is employed which is formed with an n-type cladding region 2 on one side and a p-type cladding region 3 on the other side of a p-type active region 1, respectively, said p-type active region 1 and p-type cladding region 3 being made of mutually different semiconductive materials to provide a difference .DELTA.E of band gap energy between regions 3 and 1. In such a light emitting semiconductor device, the effective height of the energy potential barrier for confined electrons or holes is the difference .DELTA.E of the band gap energies which is determined by material constants inherent in the semiconductor materials of active region 1 and p-type cladding region 3.
On the other hand, with the recent advance of crystal growing technology, such as MBE (molecular beam epitaxy) processes, MOCVD (metal-organic chemical vapor deposition) processes, etc., the crystal growth of super thin layers on the order of an atomic layer has become feasible and property control of semiconductor devices has been effected by making use of a quantum size effect. For example, in a quantum well laser with an active layer several tens of angstroms thick, reduction of the threshold current and shortening of the light emission wave-length have been contemplated with an increase in the state density function due to the dispersion of the energy levels of the electrons and holes in the quantum wells. Further, it has been proposed that, as an electron device, a CHIRP (Coherent Hetero Interface for Reflection and Penetration) superlattice be utilized which has a base region composed of two kinds of semiconductor materials different in band gap, alternately arranged with their periodicity varied little by little.
The aforementioned conventional light emitting devices have a disadvantage such that, if they are operated at high temperatures, the electrons and holes are thermally excited to high energy levels so that these carriers, such as electrons and holes, leak out over the potential barrier, whereby the light emission efficiency is noticeably lowered. They have a further disadvantage such that, when they are used as a laser, the threshold current increases. Furthermore, for example, GaInAsP lasers emitting in the wavelength region of 1.5 .mu.m have a shortcoming such that some carriers leak out at high temperatures via the Auger process so that operation at high temperature is difficult. These problems are caused by insufficient formation of effective energy potential barriers for electrons and holes.
On the other hand, with the advance of membrane forming technology, superlattice structures of various characteristics have been developed which are expected to be utilized in light emitting devices. However, the above-mentioned superlattice structures have so far been utilized only as electronic devices, and the technology for confining electrons and holes in an active region for light emitting devices by making use of superlattice structures has not yet been developed.